1. Field of the Invention
This invention relates to a process for preparing low unsaturation polyether polyols. More particularly, this invention relates to a process for the preparation of polyether polyols, such as polyoxypropylene glycols, having reduced levels of terminal unsaturation.
In the process of this invention allyl terminal unsaturation of a polyether in a first step is reduced by conversion to propenyl unsaturation by contacting the polyether with an isomerization catalyst after which a substantial amount of the propenyl terminal unsaturation is removed by contacting the polyether with an acid catalyst and the corresponding polyether having an additional hydroxyl group is obtained.
During the preparation of polyethers, such as polyoxypropylene glycols, with molecular weights greater than about 1000, monofunctional products having allyl ether terminal (CH.sub.2 .dbd.CH--CH.sub.2 --O--) groups are formed as a result of the base catalyzed rearrangement of propylene oxide to allyl alcohol and subsequent propoxylation of the allyl alcohol.
The problem of the rearrangement or conversion of propylene oxide to allyl alcohol increases substantially as the molecular weight of the polyoxypropylene glycol product increases. Thus, the problem of the formation of the monofunctional products, which is quite small in low molecular weight polyols, increases significantly as the molecular weight increases to 1000 or more.
It is typical for commercial polyoxypropylene glycol products prepared conventionally using basic catalysts, such as potassium hydroxide, to have unsaturated terminal groups in the range of about 3-4 mol percent of the total number of the terminal groups. These products having unsaturated terminal groups are undesirable for use in the preparation of polyurethanes since the unsaturated terminal groups which are unreactive toward isocyanates not only limit the molecular weight of the urethane product but, additionally, may cause deleterious physical property effects on the end product if the concentration of the unsaturated terminal groups is too high. Further, during the amination of the polyoxyalkylene glycols to form amine terminated polyols, the unsaturated terminal group is hydrogenated to a propyl group which is also unreactive toward isocyanates and may affect adversely the physical properties of the resulting polymer product. It is therefore desirable to prepare polyols with lowered levels of unsaturation to be used in preparing polyurethanes and amine terminated polyols with improved properties.
2. Prior Art
U.S. Pat. No. 5,010,187 to Heuvelsland teaches a process for preparing a polyester polyol having an equivalent weight of from about 200 to about 5000 by reacting propylene oxide with an initiator containing at least two active hydrogens in the presence of a catalyst such as barium, strontium and their oxides, hydroxides, hydrated hydroxides or monohydroxide salts or mixtures thereof. The prepared polyol has a total unsaturation level of less than 0.080 millequivalents/gram polyol.
U.S. Pat. No. 5,095,061 to Chavez, Jr., et al., teaches a process for reducing the amount of propenyl polyethers in hydroxy-functional polyethers which comprise contacting a neutral hydroxy-functional polyether containing a propenyl polyether and water and an acid catalyst soluble in the polyether, such as hydrochloric, sulfuric, phosphoric, etc., under reaction conditions sufficient to convert a portion of the propenyl polyether to propionaldehyde followed by scavenging the acid with an epoxy compound and finally removing the epoxy compound and the propionaldehyde.
U.S. Pat. No. 5,103,042 to Durvasula teaches a method for reducing unsaturation in polyethers in which a polyether having at least a hydroxyl group and allyl terminal unsaturation is contacted with an allyl complex-forming catalyst, such as palladium and a hydrogen donor, such as ammonium formate, at a temperature below that at which significant decomposition of the polyether occurs whereby a measurable portion of the allyl terminal unsaturation of the polyol is removed and the corresponding polyether having an additional hydroxyl group is obtained.
Derwent Abstract A25 94-001294/01 to EP 576132-Al discloses a process for reducing the level of unsaturation in polyoxyalkylene polyols by extracting with an immiscible low molecular weight polyol (i.e., less than 500) such as ethylene glycol, diethylene glycol, triethylene glycol, glycerol and butanediol, etc. The extraction may be carried out continuously or repeatedly (2-10 times) batchwise. The purified polyol products have superior properties to those made from untreated polyols.